It doesnt stop growing. It reaches a certain length and then it falls out. It just appears to stop growing because it is constantly replaced by new hair.
All hair has a growth phase, termed anagen, and a resting phase, telogen. On the scalp, anagen lasts approximately 3 years, while telogen lasts roughly 3 months, although there can be wide variation in these times between individuals. During telogen, the resting hair remains in the follicle until it is pushed out by growth of a new anagen hair.
Some types of hairs fall out faster than other types. For example, an eyelash lives for an average or 150 days before it falls out.
So it's not that leg or arm hairs "reach a certain length and stop". It's that they reach a certain length and have reached the end of their lives and they die and fall out, to be replaced by shorter ("younger") hairs.
Head hair lives quite a long time --- years.
Did you know each hair follicle has up to three hairs in it? Each in different stages. One being produced in the follicle, one planted firmly in the follicle and one about to fall out. The weight of the hair itself, or in the case of pubic hair, friction from undergarments "etc" will cause it to fall out.
The only hair cells that are growing are in the hair root. The cells of the hair shaft are dead. The hair shaft is the hair that you see and cut. The outer cells are made of tough proteins. This protects the hair from wearing out. The cells on the inside are stretchy. This protects them from breaking off when you comb them. Some of these proteins are high in sulfur. You can smell this burning sulfur when hair burns. A single hair has a thickness of 0.02-0.04mm.
There are no nerve cells in the hair. The hair root has no way of knowing how long the hair is. Your eyebrows are always the same length. If they are shaved they grow back to the original length and then stay that length. Your eye lashes do the same thing but they are about twice as long. The hairs on your arm or leg "know" how to grow to their proper length. To understand how this works we need to learn more about how hairs grow.
The growing phase is called anogen. The anogen phase is different lengths of time for different types of hair. Short hairs like eye brows only grow a few weeks. Your scalp hair can grow for years. 85% of your hairs are in anogen.
After the anogen phase the hair enters Catagen. Catagen is only lasts a week or 2. This is a transition period. The living cells of the hairs root slow down and change their structure. Only 1% of your hairs are in catagen.
Telogen is the final phase. Telogen is a resting phase. During telogen the hair doesn't grow. Telogen lasts a few months. 14% of hairs are in telogen. Some hairs fall out during Telogen. A few hairs don't fall out until the new hair starts to grow in the next anogen phase.
Scientists have observed hairs on different parts of the body. They find these ranges of growth for different kinds of hair.
Anagen , grows 4 to 8 years
Telogen, rests 2 to 4 months
Average growth rate (mm/day)
Eyelashes. trunk, and extremities
Anagen, grows 1 to 6 months
Telogen, rests 2 to 4 months
Average growth rate
Coarse body hairs 0.27(mm/day)
Mustaches (beards or whiskers) 0.4(mm/day)
Armpit hairs 0.3(mm/day)
Pubic hairs 0.2(mm/day
Hairs fall out at the end of telogen and the beginning of anogen. This means that on average 40 - 100 scalp hairs are entering anogen or the growth phase everyday.
It is normal to lose up to about 100 hairs a day on one's comb, brush, in the sink or on the pillow. This is the result of the normal hair growth cycle. Hairs will grow for a few years, then rest for a few months, shed, and regrow. Telogen is the name for the resting stage of the hair growth cycle.
A telogen effluvium is when some stress causes hair roots to be pushed prematurely into the resting state. Telogen effluvium can be acute or chronic.
If there is some "shock to the system", as many as 70% of the scalp hairs are then shed in large numbers about 2 months after the "shock". This sudden increase in hair loss, usually described as the hair coming out in handfuls, is acute telogen effluvium. This is a different problem than gradual genetic hair thinning. However, this can be seen in the less common chronic telogen effluvium, only after a significant amount of hair has already been lost.
A considerable number of different causes for telogen effluvium exist. Among the common causes are high fevers, childbirth, severe infections, severe chronic illness, severe psychological stress, major surgery or illnesses, over or under active thyroid gland, crash diets with inadequate protein, and a variety of medications. Most hair loss from medications is this type and causes include retinoids, beta blockers, calcium channel blockers, antidepressants, and NSAIDS (including ibuprofen).
Typically, abrupt diffuse hair loss is noticed several weeks to several months after the incident has initiated the biologic program for hair loss. While the most often noticed hair loss occurs on the scalp, some individuals may also notice hair loss elsewhere on the body. Significant hair shedding usually occurs when shampooing, combing, or even when gently manipulating the hair. Shedding usually slowly decreases over 6 to 8 months once the cause for the hair loss is no longer present. As some of the causes represent ongoing problems, it is important to determine the likely cause when possible and take appropriate measures to prevent continued hair loss.
These shed or loose hairs all have club-shaped "roots" typical of resting, telogen hairs and may be easily identified under the microscope. After shampooing, the bulk of existing loose hair has often been shed and loose hair may not again appear until additional hairs enter this resting phase. When there is any doubt about the presence of this condition, a small piece of skin may be taken from the scalp as a biopsy to be examined under the microscope. In this way, the condition of the hair follicles, the tissues that produce the hair, may be determined.
No treatment is needed for most cases of telogen effluvium. Remember that the hairs fall out when a new hair growing beneath it pushes it out. Thus with this type of hair loss, hair falling out is a sign of hair regrowth. As the new hair first comes up through the scalp and pushes out the dead hair a fine fringe of new hair is often evident along the forehead hairline.
The most important issue in telogen effluvium is to determine if an underlying cause for the problem is present. Blood tests may need to be done if the cause is not obvious, such as mild iron deficiency. If the telogen effluvium is caused by a medication, the medication needs to be stopped. When the cause of the hair loss is something like giving birth, a transient illness, or other self-limited problem the induced telogen effluvium is also usually self-limited and requires no treatment.
Chronic telogen effluvium is recently recognized and not uncommon. It often occurs in women who previously had very thick hair in their teens and twenties and still have an apparently normal head of hair to a casual observer. It affects the entire scalp with no obvious cause apparent. It usually affects women of 30 to 60 years of age, starts suddenly and has a tendency to fluctuate for a period of years. The degree of shedding is usually severe in the early stages and the hair may come out in handfuls. It does not cause complete baldness and does appear to be self-limiting in the long run.
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